Discussion - Elemental Abundances of Kepler Objects of Interest in APOGEE

Chapter VI: Elemental Abundances of Kepler Objects of Interest in APOGEE

6.5 Discussion

the most negative slope regions. The absence of this tail in the Nibauer et al. (2021) abundances, which are comparable to ASPCAP abundances, may again be due to evolutionary state differences in our respective stellar samples.

𝑇_𝑐patterns can also be examined by splitting abundances into volatile and refractory groups, and fitting individual linear trends to both sets. This was done by Bedell, Bean, et al. (2018) for a sample of solar twins (see their Figure 4). Stars with enrichment trends will exhibit steeper linear fits to abundances with𝑇_𝑐 > 1000 K compared to abundances with𝑇_𝑐 < 1000 K, while the opposite will be true for depletion trends. We carry out this analysis for our KOIs and their doppelgängers, and provide examples of our linear trend fits in Figure 6.11. Because strong enrichment results in steeper refractory trends, the linear fits will have lower intercept values.

Thus, enrichment pattern strength can be likened to the difference in volatile and refractory linear fit intercepts. We plot the distributions of these intercept differences in Figure 6.12. The distribution corresponding to ASPCAP abundances exhibits a tail towards higher intercept differences that is not present in the distribution derived from predicted abundances. We examine the ASPCAP𝑇_𝑐 trends for the KOIs with the top five largest intercept differences, and find that they have anomalously low measured [Na/H] (ranging from −0.23 dex to−1.29 dex) that are ≳0.2 dex below the other measured abundances. The associated errors on measured [Na/H] are large (0.074−0.94 dex). In addition, four out of the five KOIs with largest intercept differences overlap with the five KOIs that are outliers in predicted and ASPCAP [Na/H] space (Figure 6.5, [Na/H] KOI panel). This is further evidence that the [Na/H] intrinsic dispersion differences in the initial sample selected on [X/Fe]_error

< 0.1 dex are the result of large abundance uncertainties. We conclude that if there are underlying differences in the individual abundance𝑇_𝑐 trends for the KOI and doppelgänger samples at fixed evolutionary state, [Fe/H], and [Mg/H], they are marginal. To be detected, these differences must exceed the sensitivity of our predicted abundances, which is typically𝜎_intrinsic ≈0.038 dex and 0.041 dex for the KOIs and doppelgängers, respectively.

specifically, we compute model-measurement abundance residuals from ASPCAP and predicted abundances using a four-parameter model (𝑇_eff, log𝑔, [Fe/H], [Mg/H]), and find that there are no differences in residual structure between the KOI and dop- pelgänger samples. We calculate the median intrinsic dispersion across all analyzed elements other than (Fe, Mg) to be𝜎_intrinsic ≈0.038 dex and 0.041 dex for the KOI and doppelgänger samples, respectively, which can be taken as the minimum abundance precision required for discerning individual abundance signatures related to planet formation.

Because we do not know the planet membership of our doppelgänger sample, some doppelgänger stars may be planet hosts. This is plausible because large planet discovery surveys such as the Kepler and TESS missions have revealed that planets are common. Using Kepler DR25, Hsu et al. (2019) recently calculated an upper limit occurrence rate of 0.27 planets per star for 0.5−16 𝑅_⊕ planets around FGK dwarfs. Breaking occurrence rates by planet architectures reveals that the majority of these planets are small (𝑅 = 1−4 𝑅_⊕) and generally classified as super-Earths and sub-Neptunes (e.g., Burke et al. 2015; Zhu et al. 2018; Bryson et al. 2021).

If a significant fraction of our doppelgänger set consists of planet hosts, it makes sense that the abundance distributions of our KOI and doppelgänger samples are indistinguishable at fixed (Fe, Mg) and evolutionary state.

To reliably examine abundance differences between planet hosts and reference dop- pelgänger stars drawn from the field, none of the reference stars should host planets.

Unfortunately, constructing a sample of doppelgänger stars that we know lack planets is difficult. This would require extensive monitoring of targets with Doppler planet search surveys to ensure that there are no RV signals indicative of planets.

Carrying out such observations for an entire reference set of stars would be time and resource intensive. However, certain planet populations can be ruled out with minimal telescope time; close-in giant planets are more easily detected in RV and transit data without long cadence compared to smaller planets on longer orbits. In addition, close-in giants are intrinsically rare. RV surveys produce hot Jupiter (𝑃 <

10 days) occurrence rates of ∼0.8−1.2% around solar-like stars (e.g., Mayor et al.

2011; Wright et al. 2012; Wittenmyer et al. 2020), and transit surveys yield even smaller occurrence rates of∼0.4−0.6% (Howard et al., 2012; Fressin et al., 2013;

Petigura et al., 2018; Kunimoto and Matthews, 2020). These rates are still small for warm Jupiters (𝑃 <50 days), with estimates of∼1.3%. They remain small for hot and warm sub-Saturns (𝑅= 4−8𝑅_⊕) as well, which have occurrence rate estimates

of∼0.4% and∼2.3%, respectively (Howard et al., 2012). Thus, constructing a reference sample without close-in giant hosts is feasible. We hope to examine close-in giants in future studies, but this will require another planet host sample as only 18 of our KOIs host confirmed/candidate hot/warm sub-Saturn to Jupiter-sized planets according to the standard definition (𝑅 >4𝑅_⊕ and𝑃 <100 days).

Previous studies have found interesting abundance differences between stars that host and do not host close-in giants. For example, Meléndez et al. (2009) determined that the Sun exhibits a refractory depletion trend with𝑇_𝑐 relative to eleven solar twins from theHipparcoscatalog, as well as four solar analogs with close-in giant planets.

However, six other solar analogs lacking close-in giants as verified by RV monitoring show the solar depletion trend 50−70% of the time. One potential explanation for the solar pattern is sequestration of rocky material in the terrestrial planets, and late (10−25 Myr) accretion of dust-depleted gas once the solar convective zone began shrinking to its current mass fraction (∼2%, Hughes, Rosner, and Weiss 2007). Another explanation is that all solar twins and most solar analogs lacking close-in giants engulfed planetary material at late times (>25 Myr), once their convective zones were thin. This scenario would produce refractory enrichment in stellar photospheres. However, it assumes that most solar-like stars are depleted in refractories (at least in the absence of events like planet engulfment), and more recent abundance studies of larger Sun-like samples show that this is not the case (e.g., Bedell, Bean, et al. 2018). Either way, the findings of Meléndez et al. (2009) suggest that close-in giant planets play a role in altering host star abundances. While their results defy a clear explanation, a larger sample of close-in giant hosts and reference stars lacking close-in giants could be leveraged to examine these trends more closely.

The KOI and doppelgänger median abundance prediction intrinsic dispersions are

∼0.038 dex and∼0.041 dex, respectively. These values can be considered the upper limit of abundance precision needed to discern planet formation signatures in the elemental abundance patterns of host stars. Planet formation processes can exceed these levels in rare cases, such as the reported planet engulfment detection in the HD 240429-30 system (∼0.2 dex, Oh et al. 2018). Planet hosts may also be born with different abundances compared to stars without planets. The planet-metallicity correlation indicates that this is true for at least [Fe/H]. Such primordial abundance deviations must also exceed our intrinsic dispersion levels to be detectable.

Our KOI and doppelgänger residual abundance distributions are indistinguishable, which yields two possibilities: (1) our reference doppelgänger set includes too

many planet hosts, or (2) primordial or post-birth abundance patterns related to planet formation in our samples are below detectable levels. We can tackle the first possibility by focusing on more easily detectable planet architectures, namely close-in giants as discussed earlier. The second possibility could be addressed with higher-precision abundances from advances in spectral synthesis pipelines and/or line lists (e.g., Schuler, Flateau, et al. 2011; Liu et al. 2018; Bedell, Meléndez, et al.

2014), or from spectrographs with higher resolving power (e.g., Adibekyan, Sousa, et al. 2020). Many stars in our KOI and doppelgänger samples have abundance uncertainties that exceed our intrinsic dispersion values. Large uncertainties are the root cause of the particularly poorly measured Na abundances for the five outlier stars in our initial sample selected on [X/H]_err<0.1 dex. Upgrades to the ASPCAP pipeline, such as improved line lists and advances to the spectral synthesis pipeline, may improve APOGEE abundance precisions in the years to come.

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Dalam dokumen Planet Host Star Properties as Probes of Planet Formation and Evolution (Halaman 189-200)